The NASA objective of expanding the human experience into the far reaches of space requires the development of regenerable life support systems. This invention concerns the development of regenerable carbon sorbents for trace-contaminant (TC) removal for the space suit used in Extravehicular Activities (EVAs), and also for cabin air revitalization system. The main trace contaminant of concern is ammonia, the concentration of which should not exceed about 20 ppm. It will be appreciated by those skilled in the art that the sorbents described in this disclosure may be used in other applications where ammonia needs to be removed from a gas environment using a sorbent that can be regenerated by exposure to vacuum or a flow of purge gas.
Currently, a bed of granular activated carbon is used for TC control. The carbon is impregnated with phosphoric acid to enhance ammonia sorption, but this also makes regeneration difficult, if not impossible. Temperatures as high as 200° C. have been shown to be required for only partial desorption of ammonia on time scales of 18-140 hours (Paul, H. L. and Jennings, M. A., “Results of the trace contaminant control trade study for space suit life support development,” Proc. 39th Int. Conf. on Environmental Systems (ICES), Savannah, Ga., Jul. 12-16, 2009, SAE technical paper No. 2009-01-2370, SAE International, 2009). Neither these elevated temperatures nor the long time needed for sorbent regeneration is acceptable. Thus, the activated carbon has been treated as an expendable resource and the sorbent bed has been oversized in order to last throughout the entire mission [23 kg carbon for cabin-air revitalization and about 1 lb (0.454 kg) for the space suit]. Another important consideration is pressure drop. Granular sorbent offers significant resistance to gas flow, which is associated with a high demand for fan power. Thus, there is a great need for an effective TC sorbent that could be regenerated by short exposure to vacuum at low temperatures (under 80° C. for less than 1 hour). A monolithic structure (e.g., a honeycomb) is also desired to reduce fan-power consumption.
The current state of the art and historical approaches to trace-contaminant removal in the primary life support system (PLSS), often referred to as the space suit backpack, were recently reviewed (Paul and Jennings, supra). Activated carbon (charcoal) was identified as a clear winner for the trace contaminant control system (TCCS) application in terms of effectiveness, simplicity, and maturity of this technological solution. Carbon regeneration, however, has always been problematic, mainly because all carbons used to date were impregnated with phosphoric acid or other acidic compounds. This results in a virtually irreversible chemical reaction with ammonia and salt formation, which greatly complicates regeneration. It has been widely believed that unimpregnated carbon does not adsorb ammonia (see, for example, http://en.wikipedia.org/wiki/Activated_carbon; Luna, B., Podolske, J., Ehresmann, D., Howard, J., Salas, L. J., Mulloth, L., and Perry, J. L., “Evaluation of commercial off-the-shelf ammonia sorbents and carbon monoxide oxidation catalysts,” Proc. 38th Int. Conf. on Environmental Systems (ICES), San Francisco, Calif., Jun. 29-Jul. 2, 2008, SAE technical paper No. 2008-01-2097, SAE International, 2008; and Luna, B., Somi, G., Winchester, J. P., Grose, J., Mulloth, L., and Perry, J., Evaluation of commercial off-the-shelf sorbents and catalysts for control of ammonia and carbon monoxide,” Proc. 40th Int. Conf. on Environmental Systems (ICES), Barcelona, Spain, Jul. 11-15, 2010, AIAA technical paper No. 2010-6062, AIAA, 2010), and that chemisorption is the only option to bind ammonia to the carbon surface. It is believed that this is true only for carbons with a fairly wide distribution of pore sizes, i.e. for almost all commercial carbons. If the pore size could be optimized, however, in such a way so that almost all pores have the right size for ammonia physisorption, it is believed that no chemical impregnation is necessary to effect ammonia sorption. Furthermore, physisorbed ammonia is relatively easy to desorb using vacuum regeneration as no chemical bonds have to be broken. It is not known if any systematic studies are available that address the effect of carbon pore structure on the regeneration performance of ammonia sorbents.
It is believed that a non-optimal sorbent structure, both internal (pore-size distribution) and external (intraparticle heat transfer limitations), combined with chemical impregnation, has led to extremely long sorbent regeneration time scales on the order of 5-140 hours depending on temperature in the range 130-200° C. (Paul and Jennings, supra). The use of monolithic carbon structures for reversible ammonia sorption/desorption, as a function of temperature, pressure (vacuum), humidity, and carbon pre-treatment, is disclosed in the present specification. It will be appreciated by those skilled in the art that although the monolithic structure, e.g., a foam, is convenient from the standpoint of pressure drop, the application of this invention is by no means limited to monolithic carbon. In particular, granular carbon can be used as well. The main focus of the invention is vacuum-regenerable sorbents, but rapid resistive heating to moderately low temperatures (up to 80° C.) can also be considered as an optional feature to accelerate the vacuum regeneration process.
It is believed that good ammonia-sorption capacity can be accomplished through the combination of a particularly favorable pore structure for optimum physical adsorption (physisorption) of ammonia and carbon-surface conditioning that enhances adsorption without adversely affecting vacuum regeneration. The avoidance of acid impregnation of carbon further helps the cause of adsorption reversibility. Finally, the issues of pressure drop and fan-power requirement are addressed through the use of a monolithic sorbent structure.